SUMMARY: Thoracic aortic aneurysm and dissection (TAAD) is a life-threatening manifestation of diverse vascular smooth muscle cell (VSMC) or extracellular matrix (ECM) disorders. The histopathological hallmark of TAAD is aortic medial degeneration (AMD), characterized by proteoglycan accumulation (pooling), elastic fiber fragmentation and VSMC degeneration. Proteoglycan pools alter aorta biomechanics and facilitate dissection by raising focal tissue swelling pressure within the aortic wall but have been neglected. Their precise composition, their cellular impact and potential as a target for therapy are unexplored. We hypothesize that accumulation of the large aggregating proteoglycans aggrecan and versican is a consistent pathologic feature of TAAD, alters VSMC phenotype and survival to compromise aorta function, and could arise from reduced proteolysis. We will ask: Is aggrecan and versican accumulation a consistent feature of human TAAD regardless of primary cause, and are they indicators of disease severity? Does their accumulation result from reduced ADAMTS protease activity in addition to mRNA upregulation such as following TGF? dysregulation, commonly seen in TAAD? What are other components of the proteoglycan pools in AMD? What impact does aggrecan and versican accumulation have on VSMC? Would their reduction prevent or ameliorate TAAD? Proteoglycan profiling of the normal human aorta and TAAD aorta by tandem mass spectrometry identified 20 proteoglycans, including versican, long known to be an important vascular component, and aggrecan, traditionally regarded as a cartilage and neural component. Immunostaining of normal and TAAD ascending aortas collected prospectively at the Cleveland Clinic revealed an unexpected, massive increase of aggrecan and versican in TAAD aortas. Aggrecan accumulated in TAA in a mouse model of Marfan syndrome, and both aggrecan and versican in mice haploinsufficient for the aggrecan/versican degrading protease, ADAMTS1. The specific aims are: 1. To characterize proteoglycan dysregulation in human TAAD using quantitative tandem mass spectrometry approaches and investigate its correlation to disease severity. 2. To determine the pathogenic mechanisms by which reduced proteoglycan turnover by ADAMTS1 contributes to TAAD and the impact of excess aggrecan on VSMC function. 3. To test whether genetic deletion of aggrecan and/or versican prevents TAA and/or dissection in mouse models of Marfan syndrome and Adamts1-haploinsufficient mice. Impact: In addition to new fundamental knowledge of aortic ECM dynamics and cell-matrix interactions, the work will identify mechanisms that predispose the aortic wall to aneurysms and/or dissection, provide novel biomarkers for diagnosis and monitoring of TAAD, and identify pathways that could be targeted in non-surgical management of TAAD.